Grinding of minerals

ABSTRACT

A process for grinding a dry, or substantially dry, material in which the material is ground with a particulate grinding medium consisting of particles ranging in size from 150 microns to onefourth of an inch, the weight ratio of particulate grinding medium to material being in the range from 2:1 to 5:1. The mixture of material and grinding medium is cooled to prevent agglomeration of the particles of the material.

United States Patent lnventor Ronald Eric Broclner St. Austell, England Appl. No. 35,668 Filed May 8, 1970 Patented Nov. 30, 1971 Assignee English Clays hovering Pochin 8: Company Limlted St. Austell, Cornwall, England Priority May 15, 1969 Great Britain 24,930/69 GRINDING OF MINERALS 14 Claims, 1 Drawing Fig.

U.S. Cl 241/23 Int. Cl IB( 2 c 1 1 0 Field oiSeareli 241/15, 17,

[56] References Cited UNITED STATES PATENTS 2,565,420 8/1951 Ayers 241/15 2,764,359 9/1956 Szequari 24l/15 2,893,216 7/1959 Seefeldt et al. 241/23 X Primary Examiner-Granville Y. Custer, 5r. Attorney-Stevens, Davis, Miller & Mosher ABSTRACT: A process for grinding a dry, or substantially dry, material in which the material is ground with a particulate grinding medium consisting of particles ranging in size from 150 microns to one-fourth of an inch, the weight ratio of particulate grinding medium to material being in the range from 2:1 to 5:1. The mixture of material and grinding medium is cooled to prevent agglomeration of the particles of the material.

GRINDING or MINERALS BACKGROUND OF THE INVENTION This invention relates to the grinding of materials and, more particularly but not exclusively, is concerned with the grinding of minerals.

It is well known to grind materials to a fine particle size, for example of the order of a few microns or less, by forming a mixture of a particulate grinding medium and an aqueous suspension of the material to be ground, agitating the mixture until the material has been ground to the desired particle size, separating the particulate grinding medium from the aqueous suspension of the ground material, and drying the ground material. It has generally been considered to be essential to carry out the grinding process with the material in the form of an aqueous suspension, and this means that the suspension has subsequently to be dewatered.

It is an object of the present invention toprovide a process and apparatus for grinding a material which obviates the above disadvantages.

SUMMARY OF THE INVENTION According to one aspect of the present invention there is provided a process for grinding a material, which process comprises forming a dry, or substantially dry, mixture of the material to be ground and a particulate grinding medium, wherein the particulate grinding medium consists of particles ranging in size from 150 microns to one-fourth of an inch equivalent spherical diameter and wherein the weight ratio of particulate grinding medium to material is in the range of from 2:1 to 5:1, agitating said mixture for a time sufficient to reduce the material to the desired particle size, cooling the agitated mixture, and thereafter separating the ground material from the particulate grinding medium.

According to another aspect of the present invention there is provided an apparatus for carrying out the process of the invention, which apparatus comprises a vessel provided with an internal impeller including a rotatable shaft on which is mounted means for agitating a mixture of a material to be ground and a particulate grinding medium when the apparatus is in use, and means for cooling the mixture of material to be ground and particulate grinding medium.

The materials which are treated by the process of the present invention will either be naturally particulate or will normally be coarsely ground or crushed before being ground to a fine particle size by the process of the invention; generally, particles larger than 2.0 mm. will require coarse grinding or crushing before being treated by the process of this invention.

Processes according to the present invention are particularly useful for the grinding of minerals, for example clays and chalks.

By the term substantially dry there is meant herein that the material to be ground is sufficiently dry to flow as a powder; generally this will mean that the material will contain not more than 5 percent by weight of water but in the case of fine clays, for example, there may be present up to percent by weight of water. By grinding the material as a dry or substantially dry solid there is no need to remove water, or other suspending liquid, after completion of the grinding process.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferably, the cooling of the mixture of material to be ground and particulate grinding medium is effected simultaneously with the agitation of the mixture, for example by passing a cooling fluid through the mixture whilst the latter is being agitated.

The particulate grinding medium preferably consists of particles ranging in size from 500 microns to one'eighth of an inch, the particles having a substantially Gaussian distribution of particles size between these limits, and is advantageously formed of quartz, flint, calcined china clay, small beads of glass, or a ceramic material, for example beads of sintered alumina.

Generally, the particulate grinding medium will have a hardness in the range of from 5 to 9 on the Moh scale, and a specific gravity of at least 2.0.

The mixture of material to be ground and particulate grinding medium is cooled because attrition grinding generates a considerable amount of heat which, in the absence or substantially complete absence of water, can cause a rapid rise in temperature and this agglomeration of the fine particles. The mixture of material to be ground and particulate grinding medium can be cooled by means of a gas, e.g., air or carbon dioxide, which is injected into the mixture. Alternatively, the mixture can be cooled by injecting dry ice," ice or water into the grinding vessel in order to cool the contents thereof by evaporation, in the latter case at a rate sufficient to keep the material and particulate grinding medium cool by evaporation but no so great as to cause the material to contain more than 5 percent by weight of water at the completion of the grinding. Generally, a suitable rate will be in the range of from 0.5 to 5.0 cc. of water per minute per kilogram of material.

The ground material can be separated from the particulate grinding medium by screening the mixture on a sieve of a mesh size which will retain the particulate grinding medium and pass the ground material. Alternatively, the mixture can be fluidized in an upward flowing current of gas which will carry the ground material upwards out of the fluidized mixture leaving the particulate grinding medium behind.

If a fluidized bed separation is employed, the apparatus for carrying out the process of the invention is advantageously provided with a foraminous base, the apertures of which are sufficiently small to retain the smallest particles of the particulate grinding medium when no fluidizing gas is flowing therethrough. Generally, the width or diameter of the apertures will be in the range of from 50-500 microns. When it is desired to separate the ground material from the particulate grinding medium, a fluidizing gas is blown through the foraminous base. The velocity of gas required to achieve the separation will generally be in the range of from 10 cm./sec. to cm./sec.

The impeller used in the apparatus of the invention can be constructed from, for example, stainless steel, glass or a plastics material resistant to high temperatures, or from a metal with a coating of glass or a plastics material resistant to high temperatures. The agitating means mounted on the rotatable shaft can be, for example, a number of bars secured to and projecting from the shaft, a number of toothed discs, or a slotted cage.

The means for cooling the mixture of material to be ground and particulate grinding material can be, for example, a device for feeding a cooling fluid, generally a cooling gas, to the interior of the vessel. A suitable device is, for example, an annular chamber surrounding the vessel and communicating with the interior of the vessel through a plurality of ports in the sidewall of the vessel.

For a better understanding of the invention and to show how the same can be carried into effect, reference will now be made, by way of example, to the accompanying drawing which shows diagrammatically an elevational view, partly in section, of one embodiment of an apparatus in accordance with the invention, which can be used to grind mineral particles.

Referring to the accompanying drawing, there is shown a cylindrical vessel 1 provided with a central impeller 2 which comprises a rotatable shaft 3 to which is secured a boss 4. Projecting from the boss 4 there are a plurality of round bars 5. The boss 4 and the round bars 5 are constructed from a plastics material, e. g., poly(methyl methacrylate). The shaft 3 runs in bearings 10 and 11, and the impeller is driven by an electric motor 6 through a belt drive 7, and through pulleys 8 and 9 which can be changed to provide different speeds of rotation of the impeller. The cylindrical vessel 1 has attached to its sidewall an annular chamber 12 which communicates with the interior of the grinding vessel through a plurality of ports 13. A cooling gas is supplied to the annular chamber 12 through a conduit 14 and the flow of gas is regulated by a valve (not shown).

The invention is further illustrated by the following examples.

EXAMPLE 1 500 g. of a dry, coarse china clay were introduced into a grinding vessel, constructed according to the design described above with reference to the accompanying drawing, together with 1,500 g. of a particulate grinding medium consisting of particles ranging in size from one-eighth of an inch to 2,000 microns and comprising flint and quartz in approximately equal proportions. The particles of the particulate grinding medium had a hardness of 7 on the Mob scale and a specific gravity of 2.6. The grinding vessel was formed from poly(methyl methacrylate) and had an internal diameter of 6 inches; and the impeller was formed from the same material and had four transverse round bars for agitating the mixture of clay and grinding medium, the overall length of the bars being inches. The speed of rotation of the impeller was 1 100 r.p.m.

The impeller was rotated in the mixture of clay and grinding medium for 30 minutes during which time 55 hp.-hr. (41 kw.- hr. of energy per ton of dry clay were dissipated in the mixture. At the same time cool air was supplied to the annular chamber 12 at a rate such as to maintain the temperature of the mixture at 82 C. 180 F.) or below in order to reduce the formation of agglomerates and avoid distortion of the EXAMPLE 2 500 g. of a dry paper filler grade china clay were introduced into a grinding vessel similar to that used in example 1 but having an impeller with twelve round bars for agitating the mixture of clay and grinding material. There were also introduced into the vessel 2,000 g. of sand consisting of particles ranging in size range from 1 mm. to 0.50 mm. and comprising substantially spherical quartz grains. The sand particles had a hardness of 7 on the Moh scale and a specific gravity of 2.6.

The impeller was rotated in the mixture of clay and grinding medium for 40 minutes during which time 74 hp.-hr. (55 kw.- hr.) of energy per ton of dry clay were dissipated in the mixture. At the same time the temperature of the mixture was maintained at 82 C. 180 F.) or below by supplying a current of cool air to the annular chamber.

The clay was separated from the grinding medium by fluidizing the mixture in an upcurrent of cool air so that the particles of ground clay were elutriated out of the fluidized bed whereafter they were separated from the air in a cyclone.

The particle size distribution of the china clay before and after treatment are shown in table 1 below.

TABLE l by wt. larger by weight smaller that 10 microns than 2 microns e.s.d. e.s.d.

Before treatment 24 After treatment EXAMPLE 3 500 g. of dry, crushed coke were introduced into a grinding vessel together with 2,000 g. of the same grinding medium as was used in example 2. The grinding vessel was constructed according to the design described above with reference to the accompanying drawing but was made from brass and had an internal diameter of 5 inches. The impeller was made from stainless steel and had four round bars for agitating the mixture, the overall length of the bars being 5 inches. The speed of rotation of the impeller was l,l00 r.p.m.

The impeller was rotated in the mixture of crushed coke and grinding medium for minutes during which time 203 hp.- hr. (151 kw.-hr.) of energy per ton of coke were dissipated in the mixture. At the same time carbon dioxide gas was introduced into the annular chamber to cool the mixture to 100 C. (212? F.) or below and also to reduce the risk of combustion of the fine coke. Instead of carbon dioxide gas it is possible to use dry ice which is advantageously crushed to a particle size comparable to that of the particulate grinding medium and the particles fed to the grinding vessel continuously as the grinding progresses.

At the completion of grinding, the ground coke was separated from the grinding medium by fluidizing the mixture in an upcurrent of cool air. The coke was elutriated out of the fluidized bed and was separated from the air in a cyclone.

The coke initially had a particle size distribution such that 75 percent by-weight consisted of particles smaller than 0. l5 mm. (100 mesh B.S. sieve) and 93 percent by weight of particles larger than 53 microns (30 mesh B.S. sieve). After treatment the ground coke had a particle size distribution such that 31 percent by weight consisted of particles larger than 53 microns, 62 percent by weight of particles larger than 10 microns and 7 percent by weight of particles smaller than 1 micron.

I claim:

1. In a process for grinding a material which is in a dry, or substantially dry state, the improvement which comprises forming a mixture of the material to be ground and a particulate grinding medium wherein the particulate grinding medium consists of particles ranging in size from microns to one-fourth of an inch equivalent spherical diameter and wherein the weight ratio of particulate grinding medium to material is in the range from 2:1 to 5:1, agitating said mixture for a time sufficient to reduce the material to the desired particle size, cooling the agitated mixture to prevent the temperature of the particles of the material from rising during the agitation thereof to a level at which they tend to agglomerate, and thereafter separating the ground material from the particulate grinding medium.

2. A process according to claim 1, wherein said cooling of the agitated mixture is effected simultaneously with the agitation of the mixture.

3. A-process according to claim 1, wherein said cooling is effected by passing a cooling fluid through said mixture.

4. A process according to claim 3, wherein said cooling fluid is a gas.

5. A process according to claim 3, wherein said cooling fluid is air or a constituent thereof.

6. A process according to claim 3, wherein said cooling fluid is water or a mixture of water and a gas and wherein the water is passed through said mixture at a rate which lies in the range of from 0.5 to 5.0 cc. per minute per kilogram of said material in said mixture.

7. A process according to claim 1, wherein said cooling of the agitated mixture is effected wholly or in part by admixing comminuted solid carbon dioxide with said mixture.

8. A process according to claim 1, wherein said cooling of the agitated mixture is effected wholly or in part by admixing comminuted ice with said mixture.

9. A process according to claim 1, wherein said particulate grinding medium consists of particles ranging in size from 500 microns to one-eighth of an inch.

10. A process according to claim 1, wherein said particulate grinding medium comprises quartz, flint, calcined clay, glass or a ceramic material.

11. A process according to claim 1, wherein the ground material is separated from the particulate grinding medium by screening the mixture on a sieve of a mesh size such that the particulate grinding material is retained by the sieve and the ground material is passed by the sieve.

12. A process according to claim 1, wherein the ground material is separated from the particulate grinding material by fluidizing the mixture 'in an upward flowing current of gas, whereby the ground material is carried upwards out of the fluidized mixture leaving behind the particulate grinding material.

13. A process according to claim 12, wherein the ground material is subsequently collected by means of a cyclone.

14. A process according to claim 12, wherein said upward flowing current of gas is passed into said mixture through a foraminous plate, which plate has apertures in the range of from 50 to 500 microns, at a velocity in the range of from 10 cm./sec. to cm./sec., whereby the ground material is separated from the particulate grinding medium. 

2. A process according to claim 1, wherein said cooling of the agitated mixture is effected simultaneously with the agitation of the mixture.
 3. A process according to claim 1, wherein said cooling is effected by passing a cooling fluid through said mixture.
 4. A process according to claim 3, wherein said cooling fluid is a gas.
 5. A process according to claim 3, wherein said cooling fluid is air or a constituent thereof.
 6. A process according to claim 3, wherein said cooling fluid is water or a mixture of water and a gas and wherein the water is passed through said mixture at a rate which lies in the range of from 0.5 to 5.0 cc. per minute per kilogram of said material in said mixture.
 7. A process according to claim 1, wherein said cooling of the agitated mixture is effected wholly or in part by admixing comminuted solid carbon dioxide with said mixture.
 8. A process according to claim 1, wherein said cooling of the agitated mixture is effected wholly or in part by admixing comminuted ice with said mixture.
 9. A process according to claim 1, wherein said particulate grinding medium consists of particles ranging in size from 500 microns to one-eighth of an inch.
 10. A process according to claim 1, wherein said particulate grinding medium comprises quartz, flint, calcined clay, glass or a ceramic material.
 11. A process according to claim 1, wherein the ground material is separated from the particulate grinding medium by screening the mixture on a sieve of a mesh size such that the particulate grinding material is retained by the sieve and the ground material is passed by the sieve.
 12. A process according to claim 1, wherein the ground material is separated from the particulate grinding material by fluidizing the mixture in an upward flowing current of gas, whereby the ground material is carried upwards out of the fluidized mixture leaving behind the particulate grinding material.
 13. A process according to claim 12, wherein the ground material is subsequently collected by means of a cyclone.
 14. A process according to claim 12, wherein said upward flowing current of gas is passed into said mixture through a foraminous plate, which plate has apertures in the range of from 50 to 500 microns, at a velocity in the range of from 10 cm./sec. to 100 cm./sec., whereby the ground material is separated from the particulate grinding medium. 